An inorganic membrane may be applied, for example, as a porous coating on a porous ceramic support. Inorganic membranes offer several advantages over organic membranes. Inorganic membranes, for example, typically have high chemical and thermal stabilities that allow the membranes to be used in extreme pH and chemical environments. In addition, inorganic membranes can be easily cleaned by applying high temperature treatments such as firing.
Inorganic membranes may be used for filtration and separation applications in the environmental, biological, food and drink, semiconductor, chemical, petrochemical, gas and energy industries. These industries often require purified gas/vapor or purified liquid whose source is a mixed feed stream composed of different gas and/or liquid/particulate combinations. Specific examples include purification and separation of hydrogen gas, sequestration of carbon dioxide gas, filtration of oil/water mixtures, wastewater treatment, filtration of wines and juices, filtration of bacteria and viruses from fluid streams, separation of ethanol from biomass, and production of high purity gas and water for the semiconductor and microelectronics industry.
In the fabrication of an inorganic membrane, the porous inorganic coating layer or layers can be prepared, for example, by dipping a ceramic support into a coating slip. The coated ceramic support is subsequently withdrawn from the slip and is dried and fired. In order to obtain high flux and separation efficiency in the inorganic membrane, the pore size of the support should be as large as possible (e.g., to maximize flux), while the coating layer thereon is desirably made from inorganic particles as small as possible (e.g., to form small pores with separation or filtration functions to maximize separation efficiency). However, effectively covering large pores on a support surface with small particles can be difficult. For example, during conventional coating processes, cracks and pin-holes can be formed in the coating layer as a result of the inorganic particles partly filling pores in the support. In addition, during conventional coating processes, the coating particles tend to penetrate into the support pores instead of forming a continuous layer on the support. Particle penetration is more severe for supports with broader pore size distributions. The foregoing problems can have a negative impact on separation efficiency.
In an effort to minimize these problems, some processes include application of multiple coating layers of inorganic particles, wherein application of layers with large particles is followed by application of layers with gradually smaller particles, layer by layer. However, this process can often be inefficient in that it requires an undue number of multiple coating steps, especially when the pores of the support are more than 5 μm in size. Moreover, these multilayer coating layers may produce thick and rough layers, which can be undesirable.
Other processes attempt to modify the support surface prior to coating the surface in an effort to minimize cracks and pin-holes. For example, some processes may saturate the support with water, or with acetone as discussed in U.S. Pat. No. 4,412,921, before coating with inorganic particles. One problem with these processes is that the liquid (i.e., water or acetone) can still draw the inorganic coating materials into the pores of the support. Yet another process discussed in Kim et al., Advanced Materials, 14 (15), 2002 (1078-1081) involves pretreating a ceramic support with polyvinyl alcohol (PVA) solution. The membranes prepared from such a process still comprise pin-holes, and have discontinuous structures when the support contains pores of more than 5 μm. Processes discussed in EP 0 320 033 A1 and EP 0 524 678 A1, similarly involve techniques for modifying supports before application of inorganic coatings.
In view of the above, there is a need in the art for more favorable processes for depositing membranes of relatively small inorganic particles on supports having relatively large pore sizes or pore size distributions.